1. Field of the Invention
This invention relates to a control device for a fuel pump used for an internal combustion engine mounted on vehicles.
2. Description of Related Art
In the conventional fuel control device shown in FIG. 8, an engine control computer unit (ECU) 30 outputs a pump control signal indicating the engine drive state (vehicle speed, throttle opening angle, engine rpm, etc.) to a pump control circuit 31, and the pump control circuit 31 controls a fuel pump drive motor (pump drive motor) 32 according to the input signals. In other words, with the pump control circuit 31 the pump control signal is input to an internal logic section (signal processing circuit) 34 so that processing of the designated signal by means of an input determination circuit 35 and an output setting circuit 36 of the logic section 34 is carried out, after which the signal is sent to a drive circuit 37 to drive a FET (power MOS-FET) 38. Then, in accordance with operation of transistors T1'-T3' of the drive circuit 37, the FET 38 carries out ON/OFF drive to control current from a battery 33 to the pump drive motor 32. A constant voltage circuit 39 generates a constant voltage from the battery 33 and supplies the constant voltage to the internal logic section 34.
Furthermore, the internal logic section 34 is required in order to convert the pump control signal to a signal conforming to the actual pump drive in order to realize high-accuracy fuel pump control of recent years. In other words, if the engine control computer 30 outputs a low frequency duty signal (pump control signal) which is computed for each designated interrupt routine, the internal logic section 34 converts the duty signal into a high frequency signal. In such a case, due to pump drive based on the high frequency signal, it is possible to carry out high speed switching of the motor 32 at a cycle that is faster than the time constant of the pump drive motor 32 (response in reply to speed commands), thus achieving smooth operation of the fuel pump.
However, with the conventional fuel pump control device described above, the following problems occur due to a decline in battery voltage. In other words, with a control device of this type, as described above, signal processing of the pump control signals by the internal logic section 34 is required. However, if the battery voltage declines to a value lower than the required minimum drive voltage of the internal logic section 34, it is no longer possible to insure stable operation of the internal logic section 34. As a result, if the battery voltage declines, a situation occurs in which the pump drive motor 32 stops suddenly against the will of the driver.
Particularly when starting the engine during the cold winter period or when the battery 33 is running down, it requires time until initial combustion in the engine so that the cranking period becomes longer and the battery voltage declines considerably. In such a case, the operation of the internal logic section 34 becomes unstable so that the pump drive motor 32 stops unexpectedly, thus causing impediments to starting the engine.
Furthermore, due to a variety of engine control methods and engine specifications, a further increase in the input modes of the internal logic section 34 and more complex internal circuits can be expected. For this reason, it becomes difficult to insure stable operation of the internal logic section 34 during the decline in battery voltage.